The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/sys/seq.h

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    1 /*-
    2  * Copyright (c) 2014 Mateusz Guzik <mjg@FreeBSD.org>
    3  *
    4  * Redistribution and use in source and binary forms, with or without
    5  * modification, are permitted provided that the following conditions
    6  * are met:
    7  * 1. Redistributions of source code must retain the above copyright
    8  *    notice, this list of conditions and the following disclaimer.
    9  * 2. Redistributions in binary form must reproduce the above copyright
   10  *    notice, this list of conditions and the following disclaimer in the
   11  *    documentation and/or other materials provided with the distribution.
   12  *
   13  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   14  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   15  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   16  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   17  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   18  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   19  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   20  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   21  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   22  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   23  * SUCH DAMAGE.
   24  *
   25  * $FreeBSD: stable/12/sys/sys/seq.h 331660 2018-03-28 04:38:45Z mjg $
   26  */
   27 
   28 #ifndef _SYS_SEQ_H_
   29 #define _SYS_SEQ_H_
   30 
   31 #ifdef _KERNEL
   32 #include <sys/systm.h>
   33 #endif
   34 #include <sys/types.h>
   35 
   36 /*
   37  * seq_t may be included in structs visible to userspace
   38  */
   39 typedef uint32_t seq_t;
   40 
   41 #ifdef _KERNEL
   42 
   43 /*
   44  * seq allows readers and writers to work with a consistent snapshot. Modifying
   45  * operations must be enclosed within a transaction delineated by
   46  * seq_write_beg/seq_write_end. The trick works by having the writer increment
   47  * the sequence number twice, at the beginning and end of the transaction.
   48  * The reader detects that the sequence number has not changed between its start
   49  * and end, and that the sequence number is even, to validate consistency.
   50  *
   51  * Some fencing (both hard fencing and compiler barriers) may be needed,
   52  * depending on the cpu. Modern AMD cpus provide strong enough guarantees to not
   53  * require any fencing by the reader or writer.
   54  *
   55  * Example usage:
   56  *
   57  * writers:
   58  *     lock_exclusive(&obj->lock);
   59  *     seq_write_begin(&obj->seq);
   60  *     obj->var1 = ...;
   61  *     obj->var2 = ...;
   62  *     seq_write_end(&obj->seq);
   63  *     unlock_exclusive(&obj->lock);
   64  *
   65  * readers:
   66  *    int var1, var2;
   67  *    seq_t seq;
   68  *
   69  *    for (;;) {
   70  *            seq = seq_read(&obj->seq);
   71  *            var1 = obj->var1;
   72  *            var2 = obj->var2;
   73  *            if (seq_consistent(&obj->seq, seq))
   74  *                   break;
   75  *    }
   76  *    .....
   77  *
   78  * Writers may not block or sleep in any way.
   79  *
   80  * There are 2 minor caveats in this implementation:
   81  *
   82  * 1. There is no guarantee of progress. That is, a large number of writers can
   83  * interfere with the execution of the readers and cause the code to live-lock
   84  * in a loop trying to acquire a consistent snapshot.
   85  *
   86  * 2. If the reader loops long enough, the counter may overflow and eventually
   87  * wrap back to its initial value, fooling the reader into accepting the
   88  * snapshot.  Given that this needs 4 billion transactional writes across a
   89  * single contended reader, it is unlikely to ever happen.
   90  */             
   91 
   92 /* A hack to get MPASS macro */
   93 #include <sys/lock.h>
   94 
   95 #include <machine/cpu.h>
   96 
   97 static __inline bool
   98 seq_in_modify(seq_t seqp)
   99 {
  100 
  101         return (seqp & 1);
  102 }
  103 
  104 static __inline void
  105 seq_write_begin(seq_t *seqp)
  106 {
  107 
  108         critical_enter();
  109         MPASS(!seq_in_modify(*seqp));
  110         *seqp += 1;
  111         atomic_thread_fence_rel();
  112 }
  113 
  114 static __inline void
  115 seq_write_end(seq_t *seqp)
  116 {
  117 
  118         atomic_store_rel_int(seqp, *seqp + 1);
  119         MPASS(!seq_in_modify(*seqp));
  120         critical_exit();
  121 }
  122 
  123 static __inline seq_t
  124 seq_read(const seq_t *seqp)
  125 {
  126         seq_t ret;
  127 
  128         for (;;) {
  129                 ret = atomic_load_acq_int(__DECONST(seq_t *, seqp));
  130                 if (seq_in_modify(ret)) {
  131                         cpu_spinwait();
  132                         continue;
  133                 }
  134                 break;
  135         }
  136 
  137         return (ret);
  138 }
  139 
  140 static __inline seq_t
  141 seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
  142 {
  143 
  144         return (*seqp == oldseq);
  145 }
  146 
  147 static __inline seq_t
  148 seq_consistent(const seq_t *seqp, seq_t oldseq)
  149 {
  150 
  151         atomic_thread_fence_acq();
  152         return (seq_consistent_nomb(seqp, oldseq));
  153 }
  154 
  155 #endif  /* _KERNEL */
  156 #endif  /* _SYS_SEQ_H_ */

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